The present invention relates to a dispersion strengthened ferritic steel cladding tube for a nuclear reactor, which not only has excellent high temperature strength and swelling resistance but also is superior in compatibility with a mixed U/Pu oxide fuel (hereinafter simply referred to as the "fuel compatibility"). More particularly, the present invention concerns a dispersion strengthened ferritic steel cladding tube for use as a fuel cladding tube for a fast breeder reactor, improved, without substantial cost increase, in the fuel compatibility of the inner surface of the tube as required for prolonging the service life of fuel, and a method of producing such a cladding tube.
The fuel cladding tube for use in a fast breeder reactor serves to resist an internal pressure generated by gaseous fission products at a temperature as high as about 650.degree. C. under fast neutron irradiation and to protect a mixed U/Pu oxide fuel contained therein. Excellent high temperature strength, resistance to swelling caused by fast neutron irradiation (swelling resistance) and fuel compatibility are especially important properties required for the cladding tube.
In order to improve the economical performance of a nuclear reactor plant, it is necessary to effectuate long duration combustion of a fuel. For this purpose, development of a high performance fuel cladding tube which can endure long term service is desired.
Heretofore, as a material for producing such a cladding tube, an austenitic stainless steel, such as SUS 316 type stainless steel, having excellent high temperature strength has been used. However, it has become apparent that the austenitic stainless steel has such poor swelling resistance that it cannot endure long duration service. On the other hand, a ferritic stainless steel has markedly improved swelling resistance as compared to that of the austenitic stainless steel. However, it has a demerit that its high temperature strength is low.
As a measure for improving the high temperature strength of the ferritic stainless steel, a method, such as a mechanical alloying method, is known in which fine particles of an oxide, such as Y.sub.2 O.sub.3 or ZrO.sub.2, having excellent heat resistance are dispersed in a metal by means of powder metallurgy so as to obtain a strengthened steel. It is believed that such an oxide dispersion strengthened ferritic steel will be a prime candidate material for a long life cladding tube.
While such a dispersion strengthened ferritic steel not only has excellent swelling resistance and high temperature strength but also is desirable as a material for a long life cladding tube, however, it has the following drawbacks.
As disclosed in an example of U.S. Pat. No. 4,075,010 entitled "Dispersion-Strengthened Ferritic Alloy for Use in Liquid-Metal Fast Breeder Reactors (LMFBRS)", most of dispersion strengthened ferritic steels have a Cr content of less than 16%.
This is because if the Cr content of a ferritic steel is high, as generally known in the art, the steel undergoes 475.degree. C. brittleness and .theta. phase brittleness at a temperature of about 400.degree. to 700.degree. C. which is within the range of cladding tube operating temperatures, which causes a problem about the soundness of the tube. In order to suppress such brittlenesses, it is desired that the Cr content be about 15% or less.
Moreover, the dispersion strengthened ferritic steel has a problem of anisotropy, i.e., the high temperature strength of the steel is high along the rolling direction but low along the direction perpendicular thereto. In order to improve this peculiar anisotropy, introduction of a martensitic structure is preferred. And in order to form a martensitic structure, it is necessary that the Cr content should be limited to 13% or below. Therefore, the Cr content of the ferritic steel must be kept low.
On the other hand, the corrosion resistance, particularly oxidation resistance, of an Fe-Cr ferritic steel almost depends on the Cr content. Hence, when the Cr content is as low as 13% or below, a problem arises that the fuel compatibility due mainly to oxidation is inferior to that of austenitic stainless steels. Such a fuel compatibility, i.e., corrosion or the inner surface of a cladding tube works as a factor limiting the life and working capacity of a cladding tube made of a dispersion strengthened ferritic steel having excellent high temperature strength and swelling resistance. Accordingly, an improvement in the fuel compatibility is an important task.